Process for electrolyzing nitriles

ABSTRACT

A process of electrohydrodimerizing Alpha , Beta -mono-olefinic nitriles in a divided cell in which catholyte containing Alpha , Beta -mono-olefinic nitriles is separated from anolyte of an aqueous acid solution while reducing the concentration of hydrogen cyanide in the anolyte for preventing corrosion of the anode.

United, States Patent Inventors Aklra Yomiyama Tokyo; Shinsaku Ogawa,Miyazaki-ken; Muneo Yoshida, Miyazaki-ken; Takamasa Sakai, Miyazaki-ken,all of Japan Appl. No. 796,912

Filed Feb. 5, 1969 Patented Nov. 9, 1971 Assignee Asahi Kasei KogyoKabushiki Kaisha Kita-ku, Osaka, Japan PROCESS FOR ELECTROLYZINGNITRILES 3 Claims, No Drawings [56] References Cited UNITED STATESPATENTS 3,193,480 7/1965 Beizer et a1. 204/73 3,193,481 7/1965 Beizer204/73 3,402,! 12 9/1968 Brubaker et a1. 204/74 Primary Examiner-F. C.Edmundson Attorney-Burgess, Dinklage and Sprung PROCESS FORELECTROLYZING NITRILES BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to electrolytic hydrodimerization ofa,B-monoolefinic nitriles. More particularly, this invention relates toa process for electrolytically hydrodimerizing nitriles such asacrylonitrile while removing hydrogen cyanide produced by anodicoxidation of nitriles from an anolyte. In accordance with thisinvention, the inherently excellent resistance to corrosion of a leadperoxide anode comprising lead and silver can be preserved.

Recently, various electrolytic hydrodimerization reactions of nitrilesare being practiced. For example, the electrolyses of afi-olefmicmononitriles of the formula wherein R R, and R represent hydrogen or analkyl group having normally one-five carbon atoms, and aliphatic-01,13-olefinic dinitriles of the formula wherein R,, R and R have the samemeaning as defined above, are known well. In addition, the electrolysisof lcyano-l,3-diene is also known. Typical examples of the nitrilesknown to be electrolyzed include acrylonitrile, methacrylonitrile,crotononitrile, Z-methylenebutyronitrile, 2- pentenenitrile,2-methylenevaleronitrile, Z-methylenehexanenitrile,2,3-dimethylcrotononitrile, tiglonitrile, senecinonitrile,2-ethylidenehexanenitrile, fumaronitrile, itaconitrile, citraconitrileand butyrfumaronitrile.

in the electrolyses of these nitriles, the electrolytes contain otherthan these olefinic nitriles, derivatives thereof in which double-bondsare reduced, hydrodimers, hydrotrimers and hydrodimers-of mixtures ofthese olefinic nitriles and other olefms. In these electrolyses,nitriles such as acetonitrile byproduced may be used as solvents.

ln this invention, hereinafter, the term nitriles" is used to mean thosenitriles referred to above.

The process for electrolyzing these nitriles has been known as disclosedin US. Pat. Nos. 3,193,476; No. 3,193,479; No. 3,193,480 and No.3,193,481 and British Pat. No. 1,076,610.

Although the electrolyses of the prior arts referred to above employcatholytes in the form of a homogeneous solution, there has also beenknown an electrolysis process in which emulsified nitriles are used as acatholyte, as described in Japanese Pat. application Nos. 37988/ 1966and No. 37989/1966.

in the electrolysis of nitriles as referred to above, if nitrilesdiffuse into anolyte and are oxidized at the anode, the resultingoxidized products cause extraordinary corrosion of the anode. Forpreventing this inconvenience, there has been proposed to carry out theelectrolysis in a divided cell in which anolyte is separated fromcatholyte by a membrane as disclosed in U.S. Pat. No. 3,193,480.

However, although the extraordinary corrosion of anode may be preventedto some extent by the provision of the partition membrane, the effect isstill unsatisfactory from the practical point of view, while the use ofa membrane having porosity fine enough to completely prevent difi'usionof nitriles into an anode compartment is impracticable for economicalreason, because such a membrane will have an exceedingly high electricalresistance.

Hence, when using conventional membranes in a divided electrolysis cell,small amounts of nitriles inevitably diffuse into an anode compartmentand leads to anodie oxidation thereof at the anode. Thus, it isnecessary that either an anode I which is insusceptible to suchextraordinary corrosion due to anodic oxidation of nitriles be used, or,alternatively, the

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anolyte be subjected to a special treatment as described hereinafter.

Normally, platinum, nickel, carbon, magnetic iron oxide, lead and leadperoxide are known as materials for anodes. Among these materialsenumerated above, most preferable are lead and lead alloy on which acoating layer of lead peroxide is formed, considering its low cost,small amount of corrosion and workability and in view of the fact thatit has resistance to corrosion even in an acidic solution and has noadverse influence on hydrodimerization reaction.

It is also known from various literature that the corrosion resistancemay be enhanced by alloying lead with silver or further with antimony,or by applying heat treatments to the resulting alloys, or byincorporating thereinto small amounts of tellurium, tin, cadmium,calcium, copper and cobalt.

Among these suggested method of improving the corrosion resistance ofanodes, it has been believed that the alloying of lead with silver wasmost effective.

However, it has been found that when an electrolysis is carried out for200-500 hours in a divided cell partitioned by a membrane employing thelead-silver alloy as an anode and using a solution or emulsion ofnitriles as a catholyte and sulfuric acid, which is most preferable whenusing the lead-silver alloy anode, as an anolyte, because of anodicoxidation of nitriles which are diffused through the membrane, corrosionoccurs to create on the surface of anode quite unusual blisters of morethan 1 mm. heights which subsequently come off and the anode becomes nolonger usable. The higher the silver content becomes, the more drasticthe detrimental phenomenon prevails. This is contradictory to theaccepted theory that, in general, up until silver content exceeds 5%,the higher the silver content becomes, the more the corrosion resistancein sulfuric acid solution is increased.

In producing adiponitrile by the electrolytic hydrodimerization ofacrylonitrile in a divided cell partitioned by a membrane using alead-silver alloy anode and a 0.5N aqueous sulfuric acid as an anolyte,there are accumulated in the anolyte nitric acid ions and the amount ofcorrosion is increased. To eliminate this inconvenience, French Pat. No.1,487,571 proposes a process for removing the nitric acid ion by using aliquid ion exchange resin.

The present inventors have conducted an extensive study in an attempt toclarify the mechanism of the extraordinary corrosion of lead-silveralloy anode. As a result, it has been found that when nitriles areoxidized at anode there is produced hydrogen cyanide which isattributable to the extraordinary corrosion of lead-silver alloy anodeand that the extraordinary corrosion of anode and the formation ofnitric acid may be prevented by removing hydrogen cyanide from anolyteto reduce the concentration thereof. The present invention has its basison this novel finding.

The aeration process comprises recycling an anolyte, feeding thusrecycled anolyte wholly or partly from the top of a packed tower andblowing air into the packed tower from the tower bottom.

Considering the use of lead-silver alloy anode, easiness of removinghydrogen cyanide and cost, sulfuric acid is the most preferred anolyte.

When using a 10% sulfuric acid as an anolyte and an anode in which 0.1%of silver is alloyed, the permissible concentrations of hydrogen cyanideand nitric acid in the anolyte are about 200 p.p.m. and about 1000 ppm.respectively. The higher the silver content and the lower the sulfuricacid concentration become, the lower the permissible concentrations ofhydrogen cyanide and nitric acid become.

Although the formation of nitric acid can be suppressed by removinghydrogen cyanide from sulfuric acid, there may be used other methodssuch as substitution of a fresh sulfuric acid for a part of anolyte orremoval by using an ion exchange resin, may be conveniently adopted incombination with the operation mentioned above.

With regard to the composition of lead-silver alloy used for anodes, thecontent of silver in this alloy is from 0.05 to 5%,

and preferably from 0.05 to l.5%. The overall amount of corrosion oflead peroxide is reduced as the silver content is increased. But, if thesilver content is excessively high, the layer of lead peroxide isblistered by about I mm. due to the extraordinary corro'sion by hydrogencyanide and thus blistered layer tends to come off. From this point ofview, the higher the silver content in the alloy becomes, the lower thepermissible concentration of hydrogen cyanide in the anolyte becomes.

Antimony may be incorporated into the lead-silver alloy and the contentof antimony in the alloy is preferably from I to 10%. By theincorporation of antimony, the permissible concentrations of hydrogencyanide and nitric acid for preventing the extraordinary corrosion byhydrogen cyanide are increased.

lf antimony is incorporated, the lead-silver alloy may be hardened byquenching the alloy from a temperature of 230-250 C. to normaltemperature. By this quenching, the mechanical strength of the anodeprepared therefrom, such as hardness, flexal strength, tensile strength,etc, can be greatly enhanced. Antimony content of 2-6% affords mostsatisfactory hardening effect. Moreover, the amount of corrosion of leadperoxide in a sulfuric acid solution containing hydrogen cyanide can bereduced by about 50%, maximum.

in view of these advantages described above, lead-silver alloy intowhich is incroporated antimony is a preferable material for electrodesused in the electrolysis of nitriles.

The corrosion resistance of lead-silver alloy anode may also be improvedby incorporating less than about 1% of tellurium, tin, calcium, copper,cadmium and cobalt.

As described above, sulfuric acid is most preferable as an anolyte andthe concentration of about 530% is particularly preferred. if theconcentration of sulfuric acid is too low, not only are lowered thepermissible concentrations of hydrogen cyanide and nitric acid but alsothe electric conductivity of anolyte is decreased.

Besides sulfuric acid, solutions of phosphoric acid or aryl sulfonicacid may also be used as an anolyte.

In the process of this invention, membranes capable of preventingdiffusion of nitriles from a cathode compartment into an anodecompartment as thoroughly as possible and those having a high electricconductivity are preferred. Generally, a cation exchange membrane ispreferably used for this application.

In practising the electrolysis in accordance with this invention, ananode electric current density of about 5 to amperes per dm.particularly 5 to amperes per dm. is most preferably. The amount ofcorrosion of lead peroxide is increased if the anode current density iseither lower or higher than the range specified above.

Conventional catholyte of an aqueous concentrated solution of asupporting electrolyte represented by tetraethylammoniump-toluenesulfonate containing more than 5% of nitriles homogeneouslydissolved therein may be conveniently used. However, the processdisclosed in Japanese Pat. application Nos. 37988/66 and No. 37989/66 inwhich emulsified nitriles are used as catholytes has advantages in thatsupporting electrolytes having high electric conductivities suchtetraalkylammonium sulfate or halides can be used and that theseparation and purification of the product are simplified.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples willillustrate this invention more fully. However, it should not beconstrued that these examples restrict this invention in any manner.

Example l An alloy consisting of 0.1% silver, 6% antimony and thebalance of lead, which was hardened by quenching from 245 C. to normaltemperature was used as an anode. The same material was used as acathode. A cation exchange membrane of 1 mm. thickness obtained bysulfonating a divinylbenzenestyrene-butadiene copolymer was used as apartition membrane. An electrolysis cell partitioned into a cathodecompartment and an anode compartment by said membrane was employed. Ananolyte in the anode compartment and a catholyte in the cathodecompartment were recycled, respectively, between an anolyte tank and acatholyte tank by recycling pumps.

A 2N aqueous sulfuric acid was used as anolyte which was circulated inthe anode compartment at a rate of 20 cm./sec. to discharge fineparticles of lead peroxide having a particle size of about 50 removedfrom the anode.

An emulsion having a ratio of an oil phase to an aqueous phase of 1:3was used as catholyte. The oil phase was consisted of 20% acrylonitrile,6% adiponitrile, 10% propionitrile, 2% 2-cyanoethyl adiponitrile, 1%biscyanoethyl ether and 6% water. The aqueous phase comprised of 20%tetraethylammonium sulfate, 8% of said nitriles and the balance ofwater.

The electrolysis was carried out by maintaining the conditions set forthabove. Other conditions of electrolysis included an electrolysistemperature of 50 C., an electric current density of 10 amperes per dm.for both anode and cathode and an amount of anolyte of 500 cc./amperes.

One-fifth of the recycling flow to the anode compartment was supplied tothe top of a packed tower filled with one inch Raschig rings in a heightof 3 m. and an aeration was carried out by blowing into the packed towerfrom the bottom 10 volumes of air per a volume of gaseous oxygenproduced at the anode by electrolysis.

The electrolysis was carried out under these conditions for more than 50hours and the concentrations of hydrogen cyanide and nitric acid wereabout 50 ppm. and about 400 ppm. respectively. The electrolysis wasfurther continued for the total of 2000 hrs., but the conditions and theconcentrations of hydrogen cyanide and nitric acid remained unchanged.

During the period, amounts of lead peroxide discharged from the cellwere measured. As a result, it was found that 5-7 mglamperehour of leadwas corroded throughout the period.

After the electrolysis of 2000 hours, the anode was disassembled and thesurface was closely examined with a finding that the whole surface wasuniformly coated with a layer of lead peroxide of about 0.2 mm.thickness and no extraordinary corrosion was observed.

Comparative Example 1 Example I was repeated under the same electrolysisconditions using the same cell, electrode material, membrane, anolyteand catholyte as used therein except that no removal of hydrogen cyanideby the aeration was effected.

As a result, after the operation of 200 hours of electrolysis, theconcentrations of hydrogen cyanide and nitric acid in the anolytereached 500 ppm. and 2000 p.p.m., respectively. The amount of leadperoxide discharged from the cell during the 200 hours period was about5-7 mg./ampere-hour of operation. However, there were observed chokingin the anode compartment and decrease in the anolyte recycling amount at200th hour of operation so that the anode was disassembled to beexamined. it was found that the layer of lead peroxide on the anodesurface was peeled off in a depth of about 1 mm. and the whole surfacewas extraordinarily corroded.

Example 2 An alloy consisting of 0.3% silver, 4% antimony, 0.15%tellurium and the balance of lead which was hardened by quenching from245 C. to normal temperature was used as an anode. The same material wasused for a cathode. Other conditions were remained same as described inExample I The aeration conditions were the same as described in exampleexcept that a part of the anolyte was replaced by a fresh sulfuric acidat a rate of 2 ccJampere'hQur.

The electrolysis was carried out under these conditions and after theoperation of about hours, the concentrations of replacement of theanolyte was effected.

At the 200th hour of operation, there were observed chok- As a result,it was found that the layer of lead peroxide on the anode surface waspeeled off in a depth of about 1 mm. and the whole surface thereof wasextraordinarily corroded.

Example 3 ment by said partition membrane was employed.

A 3N aqueous sulfuric acid was used as an anolyte which was circulatedin the anode compartment at a rate of 2 m./sec. A 40% aqueous solutionof tetraethylammonium p-toluenesulfonate containing 10% each ofl-cyano-l,3-diene, methacrylonitrile and acetonitrile was used as acatholyte.

The electrolysis conditions included an electrolysis temperature of 50C., an electric current density of 30 amperes per dm. and an amount ofanolyte of 500 cc./ampere.

flow to the anode compartment was supplied to the top of a packed towerfilled with one inch Raschig rings in a height of 3 m., and an aerationoperation was carried out by blowing into the packed tower from thebottom 50 volumes of air per a volume of gaseous oxygen produced at theanode by electrolysis.

The electrolysis was carried out under these conditions described aboveand the concentration of hydrogen cyanide operation, no accumulation ofobserved and the concentration was invariably below 200 p.p.m.

During the electrolysis, amounts of lead peroxide discharged from thecell were measured and it was found that 1-2 mg./ampere-hour of lead wascorroded throughout the period.

ry corrosion was observed.

We claim:

1. A process for electrolyzing nitriles using a lead-silver alloy anodecharacterized in that said electrolysis being car- 2. A processaccording to claim I wherein said electrolysis being carried out bymaintaining hydrogen cyanide concentration lower than 200 p.p.m.

3. A process according to claim 1 wherein said electrolysis beingcarried out in a divided cell in which a catholyte containing nitrilesis separated from an anolyte of an aqueous sulfuric acid by a partitionmembrane.

2. A process according to claim 1 wherein said electrolysis beingcarried out by maintaining hydrogen cyanide concentration lower than 200p.p.m.
 3. A process according to claim 1 wherein said electrolysis beingcarried out in a divided cell in which a catholyte containing nitrilesis separated from an anolyte of an aqueous sulfuric acid by a partitionmembrane.